Tunable tracking filter

ABSTRACT

An integrated tuner circuit ( 20 ) with an arbitrary IF output. The tuner includes an integrated circuit with a fixed-frequency control loop ( 30 ) and a matched external variable capacitance C t , to achieve tracking of a tuned LC band-pass filter ( 10 ) with an arbitrary oscillator.

The present invention relates in general to an integrated tuner circuit,e.g., for use in televisions and radios, and more particularly, to thetracking of a tuner filter with an arbitrary oscillator.

Tuner technology has evolved to the point where a tuner for a televisionsignal receiver, radio, etc., can now be formed on a single integratedcircuit. Currently available integrated tuners are generally applicationspecific, i.e., the tuners are designed for operation using specificoscillator and intermediate (IF) frequencies. Thus, different integratedtuners may be required for different RF applications.

The present invention provides an integrated tuner circuit with anarbitrary IF output The tuner includes an integrated circuit (IC)control loop, and matched external variable capacitance C_(t), toachieve tracking with an arbitrary oscillator.

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIG. 1 illustrates a tuned LC band-pass filter with variable capacitanceC_(t) and fixed inductance L.

FIG. 2 illustrates a variable external load capacitance C_(t) and thetuned LC band-pass filter of FIG. 1.

FIG. 3 is a block diagram of an integrated tuner circuit including afixed-frequency control loop for filter tracking in accordance with thepresent invention.

It should be noted that the drawings are merely schematicrepresentations, not intended to portray specific parameters of theinvention. The drawings are intended to depict only typical aspects ofthe invention, and therefore should not be considered as limiting thescope of the invention.

A tuned LC band-pass filter 10 is illustrated in FIG. 1. The band-passfilter 10 comprises a variable total capacitance C_(t) and an inductivecoil L arranged in parallel. The band-pass filter 10 is tuned by atuning voltage V_(TUN), which is applied to a varicap diode (not shown)coupled to the band-pass filter 10. When L is fixed, the resonantfrequency ω_(tank) of the band-pass filter 10 is given by:ω_(tank)=ω_(LO)±ω_(IF)  (EQU. 1)or:ω_(tank) ²=1/LC _(t)  (EQU. 2)This implies that:ω_(tank) ² C _(t)=1/L=constant  (EQU. 3)which leads to:C _(t)::ω_(tank) ⁻²=(ω_(LO)±ω_(IF))⁻²  (EQU. 4)which is the frequency-dependent relation that is needed for tracking.

In a frequency-synthesized tuner that includes such a band-pass tunerfilter 10, the local oscillator frequency ω_(LO) of the tuner, which isapplied at a mixer, relates to a reference X-tal oscillator frequencyω_(xtal) via:ω_(LO) =N _(div) /M _(div)ω_(xtal)  (EQU. 5)where M_(div) is a fixed-frequency-divider ratio, and N_(div) is aprogrammable frequency divider. Given a fixed reference X-tal oscillatorfrequency ω_(xtal), then EQU. 5 implies that:ω_(LO)::N_(div)  (EQU. 6)which also implies from EQU. 1 that for zero- or low-IF:ω_(tank)=(ω_(LO)±ω_(IF))::N _(div)  (EQU. 7)

For a zero-IF tuner concept, the resonance frequency ω_(tank) of theband-pass filter 10 equals the local oscillator frequency ω_(LO) of thetuner for proper tracking. For a near-zero IF concept ω_(tank)≈ω_(LO)and consequently, from EQU. 4:C_(t)::ω_(LO) ⁻²  (EQU. 8)From EQU. 7, this leads to:C_(t)::N_(div) ⁻²  EQU. 9)

For a low-IF IC-concept (e.g., near-zero or zero-IF), the oscillator ordivided oscillator frequency can, for example, be offered via a currentsource to an external load capacitor C_(t), which is matched with thecapacitance C_(t) in the band-pass filter 10. An integrated tunercircuit 20 including the band-pass filter 10 and an external loadcapacitor C_(t) is illustrated in FIG. 2. The integrated tuner circuit20 includes an integrated circuit 22 having a control loop (not shown)for producing the tuning voltage V_(TUN). The external load capacitorC_(t) is tuned by the tuning voltage V_(TUN), which is applied to avaricap diode (not shown) being part of the external load capacitorC_(t). Defining the voltage across the external capacitor C_(t) as:u _(t)(t)=N _(div) ² U _(t)cosω_(xtal) t  (EQU. 10)it follows that:i _(t)(t)=−ω_(tal) C _(t) N _(div) U _(t)sinω_(xtal) t  (EQU. 11)By making i_(t)(t) amplitude independent of N_(div) and C_(t), then:C_(t)::N_(div) ⁻²  (EQU. 12)Thus, the capacitance the external load capacitor C_(t) and thecapacitor C_(t) in the band-pass filter 10 are both proportional toN_(div) ⁻². As such, in case of tracking between an oscillator in azero- or low-IF frequency concept and a varicap-tuned LC band-passfilter 10 with fixed L and variable C_(t) the integrated tuner circuit20 can generate a very well defined oscillator-frequency related voltageacross the matched load capacitor C_(t). Conversely, in case of notracking, the voltage will deviate from the predicted oscillatorfrequency dependent behavior. If, in that case the integrated tunercircuit 20 would generate a tuning voltage V_(TUN) for the capacitorC_(t) of the band-pass filter 10 as well as the external load C_(t), acontrol loop can be defined such that the (frequency divided) oscillatorvoltage across C_(t) behaves as needed for tracking. The control loopwill ensure that the frequency-dependent behavior for the oscillator andband-pass filter 10 is the same, which means that band-pass filter 10and the oscillator will de-tune with the same factor all the time.

In the above-described approach, it is assumed that the external loadcapacitor C_(t) is the only external load to the integrated tunercircuit 20. However, the integrated tuner circuit 20 will also addadditional capacitive load, which will cause tracking errors especiallyat the higher end of the frequency band, where C_(t) becomes small. Theadded capacitance (i.e., parasitic capacitance C_(p)) is determined bythe integrated tuner circuit 20 package as well as by on-chipcapacitance. Since the value of C_(p) can be estimated beforehand duringdesign of the integrated tuner circuit, compensation in the control loop30 can be taken into account.

The present invention provides a fixed-frequency control loop 30 (FIG.3) for tuning the capacitance C_(t) of the band-pass filter 10 such thatthe band-pass filter 10 keeps tracking with a virtually-variableoscillator frequency (i.e., a frequency that need not be present in theintegrated tuner circuit 20), after alignment (if necessary) at onefrequency point The control loop 30 is located within the integratedtuner circuit 20, while the band-pass filter 10 and external loadcapacitance C_(t) are located outside the integrated tuner circuit 20.The resonance frequency ω_(tank) of the band-pass filter 10 isapproximately equal to ω_(LO)±ω_(IF).

In view of the above analysis, the control loop 30 has been designed toproduce a signal having a relevant component given by the expression:1−(αω_(xtal) ²R² C)N²C_(t)  (EQU. 13)where α is a variable gain, ω_(xtal) is the X-tal oscillator 32frequency, R is a resistance, C is a capacitance, and N is aprogrammable value proportional to N_(div) for setting the value ofω_(LO). N_(div) has been converted into N, because N_(div) is usually a15 bit number, which enables small oscillator steps, but the band-passfilter 10 steps are allowed to be much larger and consequently N can belimited to, e.g., a seven bit word proportional to N_(div). From EQU. 7,therefore:ω_(LO)≈ω_(tank)

N_(div)≈N  (EQU. 14)Both previous equations (ω_(LO)::N_(div) (EQU. 6) andω_(tank)=(ω_(LO)±ω_(IF))::N (EQU. 7)) are valid and implicitly an ω_(LO)and ω_(IF) dependent relation between N and N_(div) follows. Byprogramming N and N_(div) accordingly, tracking is obtained. In case ofzero- or low-IF, N_(div)≈N will be sufficient for tracking.

In EQU. 13, N and C_(t) are the only oscillator frequency dependentcomponents. As such, as long as ω_(LO)::N_(div)≈N, the capacitance C_(t)will be tuned such that:1−(αω_(xtal) ² R ² C)N ² C _(t)→0  (EQU. 15)to ensure that the band-pass filter 10 keeps tracking with the desiredoscillator frequency.

In the control loop 30, the output U₀ of oscillator 32 is passed throughan analog multiplying circuit 34 of a type known in the art to produce asignal U₀N². For example, as illustrated in FIG. 3, the analogmultiplying circuit 34 may comprise two identical cascaded amplifierswith programmable gain. It should be noted that N and U₀N² may also beprovided digitally. The value N corresponding to the desired tuningoscillator frequency ω_(LO) of the band-pass filter 10 is 10 provided tothe multiplying circuit 34 via software or hardware control. The signalU₀N² is passed through adjustable gain circuit 36 into a stage 38designed to produce a signal 40 given by:−U ₀{1−αN ²(jω₀ RC−ω _(xtal) ² R ² CC _(p))}  (EQU. 16)A feedback stage 42 is provided to produce a signal 44 given by:−αN ² U ₀ {jω _(xtal) RC−ω _(xtal) ² R ² C(C _(t) +C _(p))}  (EQU. 17)In the block diagram, it is assumed that compensation for the parasiticcapacitance C_(p) of the integrated circuit 20 has been provided duringthe integrated circuit design phase and, as such, C_(p) appears in stage38 and in parallel to the external load capacitance C_(t).

The circuit analysis for deriving the expressions presented in EQUS. 16and 17 from stages 38 and 40, respectively, is assumed to be within thescope of those skilled in the art and will not be presented in detailherein. Also, it should be appreciated that the expressions presented inEQUS. 16 and 17 may be provided using analog and/or digital circuitryother than that disclosed herein and illustrated in FIG. 3.

The signals 40, 44 presented in EQUS. 16 and 17, respectively, arecombined in an adder 46, resulting in a signal 48 given by:U ₀{1−(αω_(xtal) ² R ² C)N ² C _(t)}  (EQU. 18)After mixing 50 the signal 38 with the oscillator 32 signal, andintegrating 52 to a 30 tuning voltage V_(TUN), C_(t) is controlled suchthat the expression presented in EQU. 13, namely, 1−(αω_(xtal)²R²C)N²C_(t)→0, is realized. Consequently, C_(t):: (ω_(LO)±ω_(IF))⁻², orC_(t)::ω_(LO) ⁻² for zero- and low-IF, which are the frequency dependentrelations needed for tracking.

In the present invention, the fixed-frequency control loop 30 uses thevalue N, which is approximately equal to the frequency division ratioN_(div), for oscillator tuning without using the actual oscillatorfrequency W_(LO) itself. In the high-IF case where the ratio N, used fortuning the band-pass filter IO tracking, does not correspond withN_(div) (i.e., N_(div)≠N), the band-pass filter 10 may be tuned to afrequency different from the than the desired oscillator frequency(ω_(LO). In this case, separate programming is required for N_(div) andN. However, after a single alignment, the frequency to which theband-pass filter 10 is tuned is accurately known for each value of N.The alignment may be accomplished via the variable gain α provided bythe adjustable gain circuit 36, and/or by adjusting the fixed value ofthe inductor L in the band-pass filter 10. Alternately, or in addition,for a small frequency offset between the band-pass filter 10 and theoscillator frequency ω_(LO), some mismatch can be given to the externalload capacitance C_(t) relative to the capacitance C_(t) in theband-pass filter 10. Consequently, by independently addressing thevalues for N, the band-pass filter 10 can be tuned to each wanted IFdistance from the desired oscillator frequency ω_(LO). It should benoted that for non-zero concepts, a frequency offset may also berealized by adaptation of the voltage dependency of the external loadcapacitance C_(t).

As stated above, the invention is not limited to zero- or low-IFapplications. After the single frequency alignment by, e.g., adjustingthe gain value α, with programmable N and ω_(xtal), the tuned LCband-pass filter 10 is tuned to each wanted frequency. Along this way,the band-pass filter 10 becomes a “frequency synthesized” filter, whichis locked to “virtual oscillator frequency” Nω_(xtal), since thisfrequency need not be present in the IC.

The freedom to choose the value for N allows the present invention toprovide a single integrated tuner circuit with arbitrary IF output.Supradyne, infradyne, zero-IF, up-conversion or one-oscillatorapplications can all be realized with tracking using the presentinvention. These applications may be provided “all-in-one” undersoftware control using the same integrated tuner circuit.

Several other features provided by the present invention should also benoted. For example, the parasitic capacitance C_(p), caused by theintegrated circuit 20 itself, can be overcompensated inside theintegrated circuit 20. As such, for proper tracking, a capacitor C_(p),needs to be externally connected to the integrated circuit.Advantageously, the LC band-pass filter 10 can always be designed forminimum unwanted parallel capacitance and consequently maximum frequencyrange. Further, the fixed-frequency control loop 30 may use the X-taloscillator frequency ω_(xtal) in the loop. This minimizes the risk ofinterference, since no new frequencies are introduced.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof the invention as defined by the accompanying claims.

1. An integrated tuner circuit, comprising: a tuned LC band-pass filterhaving a variable capacitance (C_(t)) and fixed inductance (L); anexternal load capacitor having a variable capacitance (C_(t)); and afixed-frequency control loop for producing a voltage (V_(TUN)) foradjusting the variable capacitances of the band-pass filter and externalload capacitor to achieve tracking of the band-pass filter with anarbitrary oscillator frequency ω_(LO).
 2. The integrated tuner circuitaccording to claim 2, wherein the fixed-frequency control loop furthercomprises a fixed-frequency oscillator and a circuit for receiving aprogrammable value N for setting the value of ω_(LO), wherein thefixed-frequency control loop adjusts the variable capacitances C_(t)such that C_(t)::(ω_(LO)±ω_(IF))⁻²:: N⁻², wherein ω_(IF) is anintermediate frequency.
 3. The integrated tuner circuit according toclaim 2, wherein the band-pass filter is tuned to each of a plurality ofdifferent IF distances from ω_(LO) by adjusting the programmable valueN.
 4. The integrated tuner circuit according to claim 2, wherein thefixed-frequency oscillator outputs a signal having a frequency ω_(xtal),and wherein the tuned LC band-pass filter is tuned to a virtualoscillator frequency ω_(LO) given by Nω_(xtal).
 5. The integrated tunercircuit according to claim 1, wherein the fixed-frequency control loopprovides compensation for parasitic capacitance (C_(p)).
 6. Theintegrated tuner circuit according to claim 5, further comprising acapacitor corresponding to the parasitic capacitance C_(p) in parallelwith the external load capacitor.
 7. The integrated tuner circuitaccording to claim 1, wherein the fixed-frequency control loop operatesto produce a signal:1−(αω_(xtal) ²R² C)N²C_(t)→0 where α is a variable gain, ω_(xtal) is afrequency of a fixed-frequency oscillator, R is a resistance, C is acapacitance, and N is a programmable value for setting the value ofω_(LO).
 8. The integrated tuner circuit according to claim 7, wherein Nand C_(t) are the only oscillator frequency dependent variables.
 9. Amethod for tracking a LC tuned band-pass filter with an arbitraryoscillator ω_(LO), wherein the band-pass filter includes a variablecapacitance C_(t) and a fixed inductance (L), comprising: providing afixed-frequency control loop for producing a voltage (V_(TUN)) foradjusting the variable capacitance C_(t) of the tuned band-pass filterand for adjusting a variable capacitance C_(t) of a load capacitor; andinputting a programmable value N into the fixed-frequency control loopfor setting the value of ω_(LO), wherein the fixed-frequency controlloop adjusts the variable capacitances C_(t) such thatC_(t)::(ω_(LO)±ω_(IF))⁻²::N⁻² wherein ω_(IF) is an intermediatefrequency.
 10. The method according to claim 9, further comprising:tuning the band-pass filter to each of a plurality of different IFdistances from ω_(LO) by adjusting the programmable value N.
 11. Themethod according to claim 9, wherein the fixed-frequency control loopincludes a fixed-frequency oscillator that outputs a signal having afrequency ω_(xtal), further comprising: tuning the band-pass filter to avirtual oscillator frequency ω_(LO) given by Nω_(xtal).
 12. The methodaccording to claim 9, wherein the fixed-frequency control loop providescompensation for parasitic capacitance (C_(p)).
 13. The method accordingto claim 12, further comprising: providing a capacitor corresponding tothe parasitic capacitance C_(p) in parallel with the load capacitor. 14.The method according to claim 9, wherein the fixed-frequency controlloop operates to produce a signal:1−(αω_(xtal) ² R ² C)N² C _(t)→0 where α is a variable gain, ω_(xtal) isa frequency of a fixed-frequency oscillator, R is a resistance, and C isa capacitance.
 15. The method according to claim 14, wherein N and C_(t)are the only oscillator frequency dependent variables.